1. Field of the Invention
The present invention relates to a fin inspection method of a heat exchanger, more particularly relates to a fin inspection method of a fin-and-tube type heat exchanger used in automotive heaters and the like.
2. Description of the Related Art
FIG. 1 is a perspective view showing a fin-and-tube type heat exchanger 10 generally used in automotive heaters and the like. FIG. 2 is an enlarged view of the fin and tube parts wherein the heat exchanger 10 of FIG. 1 is rotated 90 degrees. The heat exchanger 10 is provided with a core 11 serving as a heat exchanger part. The core 11 is provided with a plurality of tubes 12 through which a fluid serving as a heat exchange medium passes and a large number of fins 13 that are attached to the surfaces of the tubes to increase the heat transfer area. Reference numerals 14 indicate tank parts, 15 side plates, and 16 sheet metal.
The core 11 of the fin-and-tube type heat exchanger 10 is formed by a plurality of unit elements, each provided with one straight tube 12 and a fin 13 attached in a bellows-like state on its surface, regularly repeated and connected. In such a unit element, the fin 13 is comprised of a flat sheet folded into an S-shape which is repeated to form a bellows shape. The fin 13 is therefore a folded part provided with a plurality of curved parts. Defects in the tubes 12 and fins 13 of such a heat exchanger can be detected by appearance inspection with a considerable success rate. Such appearance inspection has been improved for automation, labor-saving, and raising accuracy up to now. Recently, inspection methods making use of image processing have been introduced.
As such an inspection method using image processing, the inspection method such as shown in Japanese Unexamined Patent Publication (A) No. 2005-321300 is known.
This inspection method is an appearance inspection method of a core of a heat exchanger having a repeated pattern of the two components of a tube and fin. According to this inspection method, two images are captured in order to apply a fault detection method using image processing. One of the images of the part being inspected is an image captured as a tube inspection image while controlling the illumination so that the brightness of the image of the fin part is suppressed. The other inspection image is captured as a fin inspection image by an illumination by which the fin part can be inspected.
Further, a two-dimensional Fourier transform is applied to these tube or fin inspection images to obtain inspection images at the spatial frequency domains. Next, for example, parts of the input images are utilized to prepare mask image data for samples of good parts and this data is used to remove the frequency components of the good parts from the inspection images. Then, a two-dimensional inverse Fourier transform is further applied to obtain fault detection images.
However, when using the aforementioned prior art for inspecting fins, the following problems have occurred. That is, in order to extract a fin, it was necessary to capture two inspection images at illuminations suitable for the tubes or the fins. Since the images are captured while adjusting the level of the brightness, differences in the surface conditions of a workpiece have become a cause of detection errors in inspection.
In fin inspection, transformed images of the inspection images obtained by application of a fast Fourier transform (FFT) and the transformed images of a normal fin part of the inspection image are used to find defects, so unless all of the fins of a normal fin part are at equal pitches, good precision detection of defective fins is not possible. Further, by applying an FFT to the entire core, factors leading to detection errors will occur and the amount of data will end up becoming massive. In the case of FFT analysis, the transforms have to be applied twice, for regular and inverse, or else defects cannot be detected, thus causing the processing speed to drop. When performing inspection processing using FFT analysis, judgment based on the dimensional threshold value was difficult.
As buckled fins, to be explained later, occur at equal pitches, there is the possibility that buckled fins may not be detected with good precision by the previous methods.
FIG. 3 shows examples of fin defects.
As modes of defects of the fins, as shown in FIG. 3, there are short fins, crushing, buckling, irregular pitch, erosion, residual flux, etc. The concepts of these fin defects are shown below, but these are not necessarily strict definitions.
When a distance between an end of the fins along the tube direction and sheet metal 16 is within certain constant range, the fins are classified as normal in length, while when otherwise, they are classified as fin defects called “short fins”.
Short fins occur in the case of fins having an abnormal length in the tube direction, for example, in the case where the fins are not bent at the prescribed intervals in the tube axis direction or in the case where fins of different specifications are mistakenly attached.
If there are for example 20 rows of fins, there will be side plates and packing at the first and 20th rows, thus the fin lengths along the tube direction will not be normal at the first and 20th rows of fins. Rows at which short fins have occurred, excluding the first and 20th rows, from the second to 19th rows are classified as having short fin defects. The first and 20th rows are referred to as the “outermost rows”, while the second to 19th rows excluding the outermost rows are referred to as rows other than the “outermost rows”.
“Crushing” is the bending of the upper and lower ends (the upper and lower ends in the vertical direction when the core surface shown in FIG. 2 is placed horizontally) of a fin that occurs when a fin is pressed from above by some sort of force. When the amount of crushing is large, the flow of air between the fins is hindered.
“Buckling” is a defect occurring at the end parts of the stack of fins (in the previous example, the first and 20th outermost rows). This refers to cases where a side plate is struck by something etc. causing the side plate to deform and along with this, the fins also ending up deforming.
“Irregular pitch” is an irregularity which occurs when the normal allowable range of the pitch of a fin is exceeded. Here, it is defined as what remains when other categorized defect modes (crushing etc.) are excluded.
“Erosion” is a defect in which a fin has melted from the heat in a furnace. The pitch of the fin ends up becoming larger at that portion.
“Residual flux” is the deposition of foreign matter such flux residue on a fin.
Up until now, these defects could not be automatically identified.